1. Field of the Invention
The present invention generally relates to a radio communication device and a method thereof for channel estimation, in particular, to a radio communication device and a method thereof able to adjust a pilot symbol insertion rate in channel estimation.
2. Description of the Related Art
Generally, coherent detection and differential detection are used for demodulation in digital mobile communications. In coherent detection, a received signal is detected using a reference waveform generated by a carrier recovery circuit, so the coherent detection shows good performance on bit error rate in an additive white Gaussian noise channel. On the other hand, in differential detection, instead of using a reference waveform, a signal received one or more symbols earlier is detected to obtain a phase difference, so, differential coding is necessary on the transmitting end. In addition, in the differential detection, noise is introduced also to the reference signal, therefore the bit error rate due to the additive white Gaussian noise is increased compared with that in the coherent detection. Further, the necessary information only comes from the phase difference before and after the bit sequence generated by the differential coding. As a result, although channel estimation becomes unnecessary, the quality of the received signal is worse than that in the coherent detection by 3 dB.
In a W-CDMA system, coherent rake reception is adopted since it is superior in quality of the received signal. In the coherent rake reception, channel estimation (estimation of amplitude variation and phase variation in the transmission channels) is one of the important techniques that largely affect the quality of the received signals. In the W-CDMA system, channel estimation is performed by using pilot symbols of known patterns inserted into a frame at a constant rate by means of time multiplexing. Various kinds of methods have been proposed to perform channel estimation using the pilot symbols. Below, WMSA (Weighted Multi-Slot Averaging) channel estimation is described.
FIG. 1 shows the principle of the WMSA channel estimation. Here, explanation is made about channel estimation of the n-th slot. As shown in FIG. 1, first, the modulated pilot symbols are removed from the n-th slot and those before and after the nth slot, or only from the n-th slot (in FIG. 1, the (n−1)-th slot, the n-th slot, and the (n+1)-th slot are considered). Then average is made (ξ (n−1), ξ (n) and ξ (n+1)) in each of these slots, and as a result, the instantaneous channel estimation of the n-th slot is obtained. Here, by averaging channel estimations of multiple slots, the precision of channel estimation against thermal noise and power interference is improvable. However, when fading changes rapidly, the obtained channel estimation is related to a slot at a time far from the n-slot (that is, a slot having low correlation), and fading tracking becomes difficult. So, appropriate weight factors are applied to ensure the precision against thermal noise (and interference power) and ease of fading tracking. So far, there have been proposed various kinds of methods for applying weight factors, for example, there is a method of adaptively controlling the average weight factor so that the SIRs of the received signals reach the maximum, a method of switching the weight factor for use within a number of weight factors, and a method of detecting the speed of fading evolution and switching the weight factor for use.
When the propagation environment changes slowly, the change of the channel estimation over multiple slots becomes small. However, in methods of channel estimation of the related art, the pilot symbols are transmitted at fixed intervals, so even when the change of the channel estimation is small and hence many pilot symbols are not necessary, pilot symbols are still transmitted very frequently, and consequently, there arises a problem in that the data are repeatedly transmitted.